Crazy fellows chatter endlessly (6)
I believe the answer is:
mental
'crazy' is the definition.
(I've seen this before)
'fellows chatter endlessly' is the wordplay.
'fellows' becomes 'men' (I've seen this before).
'chatter' becomes 'talk' (I've seen this before).
'endlessly' means to remove the last letter.
'talk' with its last letter taken away is 'tal'.
'men'+'tal'='MENTAL'
(Other definitions for mental that I've seen before include "Barking" , "Lament (anag) - of the mind" , "Involving the mind" , "like brain operation" , "Crazy" .)